The present invention is concerned with systems which allow the acoustic mapping, with the aid of a set of underwater buoys, of objects submerged within the sea. It relates more especially to the means for processing the reception signals from these buoys, which are generally transmitted by radio to a processing base usually situated in an aircraft.
It is known how to map objects situated within the sea, submarines for example, by dropping acoustic buoys from an airplane. These buoys may be passive and then simply receive the acoustic signals transmitted by the targets, or active and then receive the echo of an acoustic signal which they themselves have transmitted. These acoustic signals received by the buoys often form the subject of a preprocessing making it possible for example to determine the direction of the sound signal received, and possibly its distance in the case of an active buoy, and the signal thus preprocessed is transmitted by radio to the dropping aircraft. After having utilized the signals thus transmitted individually, there has very rapidly been cause to group these processings together so as to perform a summary thereof in order to determine more accurately the characteristics of the target thus detected.
Thus, a French patent filed on Mar. 13, 1992 in the name of THOMSON-CSF under No. 92 03005 and published on Sep. 17, 1993 under No. 2 688 595 has described a processing system making it possible to perform such mapping with the aid of an array of passive buoys of the type known by the name xe2x80x9cDIFARxe2x80x9d.
It is also possible to use buoys comprising active sonars such as those described in American U.S. Pat. No. 3,444,508 granted on May 13, 1969 in the name of Ernest A. Granfors and Co. Each buoy is furnished with its own code which is placed in the radio signal transmitted to the dropping aircraft. They can thus be identified safely and their position can be determined by direction finding from this airplane. Since these buoys are active, they each transmit an acoustic signal coded with respect to that of the other buoys in such a way as to be able to distinguish them on reception.
Each buoy receives the signals transmitted by the other buoys. An array forming a large distributed sonar is thus obtained, operating in multistatic mode, which can be dubbed a xe2x80x9cmacro-sonarxe2x80x9d.
This multistatic nature makes it possible among other things to more accurately locate the target, to determine its velocity and to increase the probability of detection thereof as well as the extent of the zone covered by the set of buoys.
With each recurrence of transmission of the group of buoys selected to cover a specified zone, an image, or chart, of this zone is obtained. This technique is known in the art by the term AGP standing for xe2x80x9cActive Geographic Plotxe2x80x9d. This image in fact gives a geographical representation of the position of the target, annotated with an estimate of velocity of this target.
The AGP is therefore a data processing which occurs downstream of the processing of the signal intended for extracting the basic information from the raw signals obtained from the buoys. The AGP therefore carries out a merging of the information thus obtained from this processing of the raw signals, the latter possibly arising from sensors other than those of the acoustic type, and even from sensors outside the buoys. An estimate of the various parameters of the target (position, velocity, etc.) is thus obtained, these estimates being of greater accuracy than those obtained by the other methods, by reason of an increase in the global signal/noise ratio afforded by the AGP.
The general principle of this technique of observation merging consists in performing the following operations:
segmentation of the domain of parameters to be estimated into elementary cells capable of containing the target, this corresponding to producing a grid of this domain;
for each elementary cell, calculation of the likelihood ratio relating to the set of observations made;
representation of the chart of the global likelihood of the presence of a target, and possibly thresholding of this chart if one wishes to extract a certain number of xe2x80x9ccontactsxe2x80x9d for each chart.
In the following description, use will be made of the terms defined thus:
zone of possible presence (ZPP), the geographical zone relevant to the construction of the AGP, which is limited by the range of the buoys. This zone will be defined by a square of side x km centered approximately either on the center of gravity of the polygon defined by the buoys, or on a point designated manually by the operator.
geographical cell, the basic element of the grid of the ZPP. The dimensions of this cell, a priori square, will be similar to those of the sought-after target, i.e. for example a 50 mxc3x9750 m square.
annulus of positional resolution, the set of geographical cells (distributed according to an ellipse in the bistatic case, a circle in the monostatic case) able to contribute via their positions to the construction of the signal observed at a given instant.
zone of possible velocities (ZVP), the domain of possible velocities for the target in the plane (Vx, Vy). For a submarine this domain will for example be a square of side xe2x88x9225 m/s, 0, +25 m/s.
velocity ambiguity locus, the domain of possible velocity vectors for a target. This domain is such that its velocity vectors give a theoretical Doppler shift compatible with the measured Doppler shift. These velocity ambiguity loci, or iso-Dopplar loci, are straight lines.
Although the optimal technique for estimating the parameters describing the target consists conventionally in processing the position and the velocity jointly, the invention proposes that a processing allowing successive determination of the position and then the velocity be performed. Although the signal/noise ratio thus obtained is in principle worse than in the optimal method, the invention is especially well suited to the case of a multistatic array to allow the position ambiguity and velocity ambiguity to be removed. Furthermore, the invention makes it easier to carry out checks and to obtain intermediate results usable during intermediate stages of the processing.
To do this, the invention proposes a system for acoustic mapping using underwater buoys, in which there is dispersed over a maritime zone a set of active buoys which each transmit a distinct acoustic signal whose echoes on a target are received by at least two of these buoys and are forwarded by radio to a processing base which extracts basic information from these signals and compiles an AGP-type chart giving a geographical representation of the position of the target, annotated with an estimate of its velocity, characterized mainly in that in this processing, use is made of a first algorithm which comprises the following steps:
producing a grid of the zone of possible presence, the so-called ZPP, as geographical cells;
creation of blind elliptical zones by deletion of the signals corresponding to the direct paths between buoys of the acoustic waves;
determination of the observed amplitude and of the observed azimuth, of the azimuth of the relevant cell and of the deviation in azimuth between the above two;
calculation of the likelihood index of the cell;
first looping of these above two steps over all the ZPP cells;
calculation of a neutral index;
second looping of the above step over all the cells of the ZPP;
normalization by this neutral index, then base adjustment;
third looping of the above step over all the cells of the ZPP;
multiplication of the likelihood indices for the various observations;
fourth looping of the above step over all the ZPP cells;
fifth looping from the step of multiplying the indices to the stop following that of the creation of the blind elliptical zones, over the set of observations; and
thresholding of the positions thus obtained so as to obtain a number N of noncontiguous contacts, forming the AGP chart of the positions; N being fixed by the processing capabilities of the algorithms for utilization according to same.
According to another characteristic, use is made of a second algorithm which follows the first and which comprises the following steps:
determination of the velocities with respect to each buoy from the successive determinations over time of the positions of targets;
producing a grid of the zone of possible velocities as so-called ZVP velocity cells;
calculation of a likelihood index for the velocities;
multiplication of the likelihood indices for the various observations;
first looping of these above two steps over the set of observations;
second looping of these above three steps over all the cells of the ZVP; and
thresholding of the velocities thus obtained and extraction of the maximum so as to determine the most likely velocities, which form the ZVP chart of velocities.
According to another characteristic, the first algorithm is performed over a set of Doppler channels so as to obtain N images AGPdop, and in that use is made of a third algorithm which follows the first and which comprises the following steps:
producing a grid of the zone of possible velocities as so-called ZVP velocity cells;
determination of the theoretical velocity associated with the relevant cell;
determination of a velocity likelihood index for each Doppler channel and for each buoy;
summation of these likelihood indices;
first looping of these above two steps over the various relevant Doppler channels;
second looping of these above two steps over the various pairs of buoys;
third looping of these above three steps over all the cells of the ZVP; and
thresholding of the velocities thus obtained and extraction of the maximum so as to determine the most likely velocities, which form the ZVP chart of velocities.